Effect of iron on the expression of sirR and sitABC in biofilm-associated Staphylococcus epidermidis

In vitro in a Fe-limited environment, the planktonic form of S. epidermidis produces siderophores and grows slower than in Fe-rich environment. The expression of sirR in planktonic bacteria, in vitro, was not different in medium without Fe or with 1 μM FeCl3. High Fe concentrations (25 μM FeCl3) increased expression of sirR transiently during the early phase of incubation. Expression of sitC in vitro, in planktonic bacteria, was inversely correlated with sirR expression in medium with 25 μM FeCl3: sitC expression decreased for the first 3 hours followed by an up regulation.In sessile bacteria in vitro, sirR expression was high and independent of the Fe concentration. The expression of sitC was not inversely correlated to sirR expression.In vivo, expression levels of sirR and of sitABC were high during the initial phase after implantation and, after a transient decrease, remained stable over a period of two weeks.Our data suggest that the expression of sirR and the regulatory effect of sirR on the sitABC operon are different in planktonic and sessile bacteria.The free ionic iron (Fe) concentration in the human body is kept at a very low level to limit bacterial growth. This is achieved through Fe-carrier proteins like transferrin and lactoferrin that bind ferric Fe (Fe3+) with a high affinity. Pathogenic bacteria however have developed powerful mechanisms that are capable of chelating Fe at very low concentrations.Coagulase negative staphylococci (CoNS) and in particular S. epidermidis are the most common cause of foreign body associated infections (FBI) such as infections of prosthetic valves, pacemakers, orthopedic prostheses and cerebrovascular shunts. A typical aspect of these FBI are the so-called biofilms consisting of multilayered clusters of CoNS that are attached to the hard surface and embedded in a slime layer [1,2].Limited data are available regarding Fe-scavenging mechanisms in Staphylococcus epidermidis. A cell surface Fe-receptor to obtain Fe from the